1. Field of the Invention
The invention relates to a frictional transmission (or tractional) apparatus which comprises a simple planetary roller mechanism having friction (or traction) rollers including a sun roller, a planetary roller (or rollers), and a ring roller, and a brake mechanism for braking rotation of these friction rollers. The invention also relates to a frictional transmission type rotational driving apparatus having the frictional transmission apparatus and a motor in combination, a series of the same, and a method of fabricating the frictional transmission apparatus.
2. Description of the Related Art
Conventionally, frictional transmission apparatuses comprising a simple planetary roller mechanism are known widely.
FIG. 8 schematically shows an example of such a conventional frictional transmission apparatus. This frictional transmission apparatus 1 comprises a simple planetary roller mechanism 10 arranged in an outer casing 9 (general view omitted). The simple planetary roller mechanism 10 has friction rollers including a sun roller 2, a planetary roller 6, and a ring roller 8. The planetary roller 6 makes rolling contact with the outer periphery of the sun roller 2 and with the inner periphery of the ring roller 8. The ring roller 8 is restricted in its rotation. Besides, a supporting pin is inserted through the planetary roller 6 so as to couple a carrier 4 to this simple planetary roller mechanism 10.
In this frictional transmission apparatus 1, the sun roller 2 and the carrier 4 make input/output elements, and the ring roller 8 a fixed element. Making the input element out of the sun roller 2 provides a speed reducing function, and making the output element out of the sun roller 2 a speed increasing function. Here, the ring roller 8 is fixed onto the part of the outer casing 9.
FIG. 9 shows the embodiment of the structure in FIG. 8, or a rotational driving apparatus 100 of frictional transmission type devised by the present inventors (not yet known publicly as of this point of application). This rotational driving apparatus 100 comprises a simple planetary roller mechanism 28 and a motor 126. The simple planetary roller mechanism 28 has friction rollers consisting of a sun roller 30, a planetary roller (or rollers) 34, and a ring roller 36. The planetary roller 34 is retained by a carrier 32, and makes rolling contact with the outer periphery of the sun roller 30 and with the inner periphery of the ring roller 36. The ring roller 36 is restricted in its rotation. The motor 126 is coupled to the sun roller 30 to drive this sun roller 30.
Specifically, a motor shaft 126A of the motor 126 is coupled to the sun roller 30 via a parallel key. The motor 126 itself is fixed to a flange portion 38B of a casing 38 which accommodates the simple planetary roller mechanism 28.
This casing 38 has a ring-shaped fixing portion 38A projected radially inward from its inner-peripheral side. The ring roller 36 is fixed to this fixing portion 38A by a bolt 40.
In the simple planetary roller mechanism 28, the sun roller 30 makes an input element, the carrier 32 an output element, and the ring roller 36 a fixed element. The entire mechanism thus has a speed reducing function. That is, this rotational driving apparatus 100 has a structure of transmitting rotational power of the motor 126 to the sun roller 30 and outputting the same from the part of the carrier 32 with a predetermined reduction ratio.
The simple planetary roller mechanism 28 transmits the rotational power from the motor 126 by means of frictional forces or oil shearing forces (tractional forces) occurring between the respective contact surfaces of the friction rollers (tractional transmission). This allows smoother, quieter operation as compared with geared and other transmission structures.
Accordingly, in order to secure a predetermined transmission capability (transmittable torque) from the simple planetary roller mechanism 28, sufficient frictional forces need to be produced between the individual friction rollers. The frictional forces are generally given by making the inside diameter of the ring roller 36 smaller than the sum of twice the diameter of the planetary roller 34 and the diameter of the sun roller 30, i.e., applying a squeezing force to the ring roller 36 for elastic deformation. The larger the squeezing force, the larger the frictional forces between the friction rollers become, and these frictional forces in turn increase the transmittable torque of the simple planetary roller mechanism 28. In contrast, smaller squeezing forces lower the transmittable torque. Incidentally, when the transmittable torque is higher (the squeezing forces are larger), the individual friction rollers increase in rotational resistance. This can lower the transmission efficiency of the rotational driving apparatus 100 and produce a low-durability problem of the simple planetary roller mechanism 28 due to rolling fatigue.
The rotational driving apparatus 100 mainly receives two merits from the adoption of the above-described simple planetary roller mechanism 28. First, as stated already, power transmission can be performed more smoothly and quietly than in geared and other transmission structures. The result is that the efficiency of the rotational power transmission is enhanced to suppress the power consumption of the motor 126, contributing to so-called xe2x80x9cenergy saving.xe2x80x9d Second, since the friction rollers are relatively easy to fabricate as compared with gears and the like, the fabrication costs are reduced to make the entire rotational driving apparatus 100 xe2x80x9clow-priced.xe2x80x9d
Meanwhile, the adoption of this frictional transmission type simple planetary roller mechanism 28 brings about the following demerit. That is, due to the structure of transmitting rotational power by means of frictional forces, the respective contact surfaces of the rotating friction,rollers always bear some slight slips therebetween. This precludes a xe2x80x9cstrictxe2x80x9d reduction ratio being maintained between the revolution speed of the sun roller 30 and the revolution speed of the carrier 32 all the time. Therefore, as long as this point is concerned, geared and other transmission structures are superior.
Considering actual use situations of such a motor in a reduction mechanism (rotational driving apparatus), however, the induction motor adopted involves a certain slip between its rotor and rotating field in the first place. Therefore, a strict reduction ratio need not be required of the reduction mechanism itself. That is, maintaining a coupled mating machine at a predetermined revolution speed typically requires measuring the mating machine or the like for revolution speed and performing feedback control onto the motor. Thus, xe2x80x9cenergy-savingxe2x80x9d and xe2x80x9clow-pricedxe2x80x9d simple planetary roller mechanisms better fit the current market needs than geared reduction mechanisms which are expensive through strict in reduction ratio.
Conventionally, there are many gear transmission type rotational driving apparatuses (so-called geared motors) with a brake mechanism added to their motor and the like. To the present inventors"" knowledge, however, neither a frictional transmission type rotational driving apparatus with a brake mechanism exists on the market, nor a proposal to add a brake mechanism has been made under current conditions. The reason for this seems that there actually exist a number of difficulties as follows.
To be more specific, frictional transmission apparatuses with a simple planetary roller mechanism transmit power by means of frictional forces occurring between the respective contact surfaces of their friction rollers, and therefore a large rotational load applied thereto can easily produces a xe2x80x9cslipxe2x80x9d on the contact surfaces. Meanwhile, brake mechanisms in common use convert kinetic energy into thermal energy by means of friction between their brake ring and brake shoes. In other words, the braking forces are produced from a xe2x80x9cslipxe2x80x9d between the brake ring and brake shoes.
Since frictional transmission apparatuses and brake mechanisms both involve potential slips, optimum combinations thereof are hard to find. Moreover, in a sense, even a lot of experience and skill cannot always ensure optimum braking characteristics for a simple planetary roller mechanism to be obtained from brake mechanisms.
For example, when braking torque obtainable from a brake mechanism is too low, it is impossible to brake a frictional transmission apparatus or a mating machine (load) coupled thereto with reliability. Therefore, the obtainable braking torque (braking capability) needs to be set considerably higher for safety. Too high braking torque, however, not only makes the brake mechanism""s power excessively high (in relation to required power) to cause a needless increase in costs,-but also leads to a hard brake, which gives rise to a problem that it becomes impossible for the friction surfaces of the individual rollers in the frictional transmission apparatus to hold the inertial force (reaction) from the mating-machine (load) side. That is, the frictional transmission apparatus can possibly produce a slip because of too high braking torque although proper braking would be possible if the braking torque were adequate and the braking were made at appropriate speed. Once a slip occurs, needless to say, it becomes difficult to apply a brake on the frictional transmission apparatus.
Accordingly, the difficulty in this type of design lies in that simply upsizing the brake mechanism for the sake of safety is not sufficient.
What complicates the designing more is the fact that the transmittable torque (maximum transmission torque) of a frictional transmission apparatus depends on the amount of interference and the like, and the transmittable torque can vary considerably with even a slight deviation of the actual interference from an interference design because of product variations.
This means too many uncertainties or unclear factors in combining a brake mechanism with a frictional transmission apparatus. Therefore, the development up to a xe2x80x9cproductxe2x80x9d requires efforts of making a number of prototypes and conducting test runs for every possible situation so as to bring the transmission capability of a frictional transmission apparatus and the braking capability of a brake mechanism into optimum range of balance.
Furthermore, when a company introduces to the market this type of frictional transmission apparatus with a brake mechanism as its xe2x80x9cproductxe2x80x9d, the introduction typically needs to involve apparatuses of various transmission capacities or various reduction ratios in a series (product group). Here, unless certain methods of designing and fabrication are developed, it is close to impossible to construct the series with overall consistency (merely by trial and error on each model).
These circumstances add up to a cost disadvantage as compared with the construction of a series from those products including gear mechanisms or other power transmission apparatuses and built-in brake mechanisms. This cost disadvantage seems to be the reason why there has been no product of a frictional transmission apparatus with a built-in brake mechanism. This is because it is generally more reliable and less expensive to use gear mechanisms with known characteristics (with no slip) rather than to use roller mechanisms one or two ranks greater than required sizes.
Nevertheless, frictional transmission apparatuses still have excellent properties such as quietness and coaxiality, and many advantages of having compact configuration, being relatively higher in reduction ratios, being easy to obtain an arbitrary value of reduction ratio, and the like.
The present invention has been achieved in view of the foregoing, and an object thereof is to provide a frictional (or tractional) transmission apparatus which comprises a brake mechanism having substantially optimum braking characteristics with respect to its simple planetary roller mechanism, and a method of fabricating the same. Another object of the present invention is to provide a frictional transmission type rotational driving apparatus having the frictional transmission apparatus and a motor in combination, and a series of the same.
The foregoing objects of the present invention has been achieved by the provision of a frictional transmission apparatus comprising a simple planetary roller mechanism and a brake mechanism, the simple planetary roller mechanism having friction (or traction) rollers including a sun roller, a planetary roller (or rollers), and a ring roller, the planetary roller being retained by a carrier and making rolling contact with the outer periphery of the sun roller and with the inner periphery of the ring roller, the brake mechanism being coupled to the sun roller to brake rotation of this sun roller. Here, static friction torque Y obtainable from the brake mechanism is set to fall within the range of 0.1Xd less than Y less than 0.7Xd, where Xd is critical test torque, a value of test torque at which any of the friction rollers starts to make sliding rotation when the test torque is input in gradually increasing values to the sun roller with the carrier and the ring roller fixed stationary.
The present inventors have first decided to introduce the following structure it a frictional transmission apparatus. That is, a sun roller makes an input element and either a carrier or a ring roller makes an output element to exercise a speed reducing function, and a brake mechanism coupled to the sun roller applies a brake on the rotation of an external load (mating machine) coupled to the carrier or ring-roller side. The reason is that this structure maintains coaxiality between the input and output shafts, is excellent in quietness, can achieve higher reduction ratios with compact configuration, and has high versatility as a most typical structure for frictional transmission apparatuses.
Now, due to its high versatility, this frictional transmission apparatus finds a great variety of uses, and can be used with various mating machines. Accordingly, the frictional transmission apparatus needs to combine a brake mechanism whose characteristics are most appropriate under various possible situations. On this account, the present inventors have decided to create virtual situations in which the simple planetary roller mechanism undergoes braking. To be more specific, suppose here that the sun roller is completely fixed by a brake mechanism, and a mating machine (external load) coupled to the carrier or the ring-roller side is maintained stationary by that brake mechanism via the simple planetary roller mechanism. Reaction torque occurring on the brake-mechanism side from the rotational power of the mating machine itself (produced by an inertial force, its own weight, and the like) then corresponds to the torque obtained by the brake mechanism. The present inventors have found that this reaction torque can be utilized to bring the transmission capability of the simple planetary roller mechanism and the braking capability of the brake mechanism into optimum balance. That is, in the present invention, this reaction torque is gradually increased in terms of xe2x80x9ctest torquexe2x80x9d application to the sun roller so that it is watched for the value (hereinafter, referred to as critical test torque Xd) actually applied at the instant when any of the friction rollers starts to make sliding rotation resultingly.
These situations, of course, will not match all the situations in which the braking torque from the brake mechanism actually puts a brake on a mating machine via the frictional transmission apparatus. However, a great number of check tests have produced the following finding. When the static friction torque Y from a brake mechanism is set to fall within the range obtained by multiplying the critical test torque Xd by predetermined factors (0.1Xd less than Y less than 0.7Xd; and favorably 0.2Xd less than Y less than 0.5Xd), it is possible to set the brake mechanism appropriately while assuming actual use situations with high reproducibility in the cases of connection with various loads and with various drive sources (e.g., a motor).
More specifically, when the static friction torque Y from the brake mechanism is set in the above-mentioned range with respect to the critical test torque Xd, it is possible to avoid almost all such situations that the capability of the brake mechanism is so high as to cause a so-called hard brake under which xe2x80x9cthe simple planetary roller mechanism starts to slip before the limit of the braking capability ensured by the brake mechanism is reached.xe2x80x9d
That setting can also preclude almost all xe2x80x9cbraking failuresxe2x80x9d even when the driving capability (driving torque) of a drive source is so high that it practically reaches the limit of the frictional transmission apparatus.
In the frictional transmission apparatus comprising a brake mechanism set as described above, a motor having a motor shaft consisting of a coupling shaft for coupling the sun roller and the brake mechanism may be installed around the coupling shaft so that rotational power from the motor is transmitted to the coupling shaft. This constitutes a frictional transmission type rotational driving apparatus.
Moreover, a plurality of frictional transmission type rotational driving apparatuses fabricated on the basis of this design concept, differing from each other in transmission capacity and in speed change ratio may form a series of frictional transmission type rotational driving apparatuses.
Now, the above-described theory devised by the present inventors has also resulted in a specific method of fabricating the frictional transmission apparatus described above.
To be more specific, test torque is initially input in gradually increasing values to the sun roller with the carrier and the ring roller fixed stationary, and the test torque is measured for the value (critical test torque) Xd at which any of the friction rollers starts to make sliding rotation.
Next, the braking portion of the brake mechanism is set and fabricated so that its static friction torque Y falls within the range of 0.1Xd less than Y less than 0.7 Xd (favorably 0.2Xd less than Y less than 0.5Xd) with respect to the critical test torque Xd measured.
According to the method of fabricating a frictional transmission apparatus comprising the foregoing two steps, it is possible to obtain an optimum combination of a simple planetary roller mechanism and a brake mechanism, allowing the fabrication of a frictional transmission apparatus with ensured braking capability and excellent cost performance. This is ascribable to the fact that not only the design theory but also the concept of virtually created use situations (braking situations) is introduced to the fabrication processes of the frictional transmission apparatus.
In this connection, the simple planetary roller mechanism described above generally has power transmission modes involving the fixed-input-output relationships as follows:
1) With the sun roller as the input element, the ring roller makes the fixed element and the carrier retaining the planetary roller the output element, or the ring roller makes the output element and the carrier retaining the planetary roller the fixed element;
2) With the carrier retaining the planetary roller as the input element, the ring roller makes the fixed element and the sun roller the output element, or the ring roller makes the output element and the sun roller the fixed element; and
3) With the ring roller as the input element, the carrier retaining the planetary roller makes the fixed element and the sun roller the output element, or the carrier retaining the planetary roller makes the output element and the sun roller the fixed element.
The simple planetary roller mechanism in the present invention adopts the mode (1) mentioned above. The present invention is applicable with either of the ring roller and the carrier as the output element.
By the way, the present inventors have introduced the new concept of xe2x80x9ctest torque inputxe2x80x9d to the method of fabricating a frictional transmission apparatus, as mentioned above. The test torque is directly input to a real (actual) simple planetary roller mechanism, and the resultant data are not of theoretical values. The introduction thereof to the fabrication processes thus makes it possible to overcome even in-process variations, so as to ensure the fabrication of a frictional transmission apparatus in optimum balance.
In a second aspect, the present invention achieves the foregoing objects by the provision of a method of fabricating a frictional (or tractional) transmission apparatus comprising: a simple planetary roller mechanism having friction (or traction) rollers including a sun roller, a planetary roller (or rollers), and a ring roller, the planetary roller being retained by a carrier and making rolling contact with the outer periphery of the sun roller and with the inner periphery of the ring roller, the ring roller applying a squeezing force to the planetary roller and the sun roller; and a brake mechanism coupled to the sun roller to brake rotation of the sun roller. Here, the squeezing force from the ring roller is set so that critical test torque falls within the range of 1.4-10.0 times static friction torque obtainable from the brake mechanism, where the critical test torque is a measurement of test torque at which any of the friction rollers starts to make sliding rotation when the test torque is input in gradually increasing values to the sun roller with the carrier and the ring roller fixed stationary.
Specifically, the actual data collection based on the xe2x80x9ctest torque inputxe2x80x9d is conducted in a manner as follow:
Initially, test torque is input in gradually increasing values to the sun roller with the carrier and the ring roller fixed stationary. Then, at first, frictional forces arising among the friction rollers produce reaction torque so that the test torque keeps increasing while the individual friction rollers are stationary. When the reaction torque produced by the frictional forces reaches its limit, one of the friction rollers makes sliding rotation so that the reaction torque drops suddenly. The value of the test torque having been actually applied until the reaction torque drops suddenly is grasped as the above-mentioned xe2x80x9ccritical test torque.xe2x80x9d
As stated previously, the mode of detecting the xe2x80x9ccritical test torquexe2x80x9d will not match every situation in which the braking torque from the brake mechanism actually puts a brake on a mating machine via the frictional transmission apparatus. Nevertheless, the present inventors have found through many check tests that multiplying this value by certain factors can provide indexes even effective for static friction torque from the brake mechanism. More specifically, the present inventors have found through a number of check tests that as long as the critical-test torque falls within the range of 1.4-10.0 times the static friction torque from the brake mechanism, this versatile frictional transmission apparatus is well applicable to almost any use mode while securing high-efficient transmission capability and braking capability. It is this finding that has led to the contrivance of the second aspect of the present invention.
To be more specific, a frictional transmission apparatus fabricated through the foregoing fabrication steps has critical test torque that falls within the range mentioned above. This means a frictional transmission apparatus with the braking capability of its brake mechanism and the transmission capability (may also be considered as retaining capability, in this case) of its simple planetary roller mechanism in balance. For example, it is possible to prevent the simple planetary roller mechanism from being relatively too small in transmission capability with respect to the braking capability of the brake mechanism. This can avoid such cases that the brake-mechanism side exerts sufficient braking torque while too small transmission capacity causes a slip in the simple planetary roller mechanism to preclude the braking torque being transmitted to a mating machine with reliability. Besides, it is possible to prevent the simple planetary roller mechanism from being so high in transmission capability (so high in rotational resistance) with respect to the brake mechanism as to produce a great (unnecessary) energy loss. Moreover, the durability of the simple planetary roller mechanism can also be prevented from dropping excessively.
As a consequence, according to this fabrication method, a frictional transmission apparatus for transmitting power to a mating machine with reliability can be obtained. Moreover, it is possible to conduct power transmission performance tests and braking performance tests on an in-fabrication frictional transmission apparatus after their systematization into a manual. Therefore, in-fabrication and design errors are prevented for a large reduction in fabrication costs, as compared with the cases where frictional transmission apparatuses are fabricated based on experience and skill before they are measured for power capacity and the like on every possible situation.
In the method of fabricating a frictional transmission apparatus described above, the critical test torque is more favorably set to fall within the range of 2.0-5.0 times the static friction torque from the brake mechanism.
Furthermore, in a third aspect, the present invention achieves the foregoing objects by the provision of a frictional (or tractional) transmission type rotational driving apparatus comprising a simple planetary roller mechanism and a motor, the simple planetary roller mechanism having friction (or traction) rollers including a sun roller, a planetary roller, and a ring roller, the planetary roller being retained by a carrier and making rolling contact with the outer periphery of the sun roller and with the inner periphery of the ring roller, the motor being coupled to the sun roller to drive the sun roller. A brake mechanism is added to the motor. Here, Y less than Xd holds, where Y is static friction torque obtainable from the brake mechanism, and Xd is a value of test torque (critical test torque) at which any of the friction rollers starts to make a slip when the test torque is input in gradually increasing values to the sun roller with the carrier and the ring roller fixed stationary. In addition, 0.65Ys less than Xs less than 3.4Ys holds, where Ys is dynamic friction torque obtainable from the brake mechanism at a rated revolution speed of the motor, and Xs is limit transmission torque transmittable by the simple planetary roller mechanism through the sun roller when this sun roller is rotated at the rated revolution speed.
As mentioned previously, the present inventors, in adding a brake mechanism to a frictional transmission type rotational driving apparatus having a simple planetary roller mechanism and a motor in combination, have given consideration to possible situations for the actual use of the rotational driving apparatus. This third aspect of the present invention focuses on a difference between the braking states at rest and at revolutions.
The gist of the invention in this third aspect is as follows:
Simple planetary roller mechanisms transmit power by means of frictional forces (or oil shearing forces: traction forces) arising between the respective contact surfaces of their friction (or traction) rollers, and thus produce a xe2x80x9cslipxe2x80x9d on the contact surfaces when subjected to a heavy rotational load. Meanwhile, brake mechanisms in common use convert kinetic energy into thermal energy also by means of friction caused by squeezing brake rings with their brake shoes. In other words, brake mechanisms generate braking power through a xe2x80x9cslipxe2x80x9d between their brake rings and brake shoes.
Since both frictional transmission apparatuses and brake mechanisms involve potential slips, it is difficult to find optimum combinations thereof in terms of sure braking. Therefore, in actual designing and fabrication heretofore, the safety to include such uncertain factors requires to set somewhat higher to set the simple planetary roller mechanisms greater in transmittable torque which depends on squeezing force.
The reason for this is that it has been considered impossible to put a sure brake on a mating machine (external load) coupled to a rotational driving machine when, for example, the simple planetary roller mechanism is too low in transmission capability (transmittable torque) with respect to the braking capability of the brake mechanism (the braking torque occurring in the braking).
On the other hand, simple planetary roller mechanisms need to transmit rotational driving power from a motor with high efficiency.
That is, when a simple planetary roller mechanism is designed and fabricated in greater dimensions (including transmission capability), the friction rollers thereof increase in rotational resistance, losing the merit of xe2x80x9ctransmitting rotational power with high efficiencyxe2x80x9d inherent in frictional transmission type simple planetary roller mechanisms. This also entails an increase in costs. Therefore, in terms of transmission efficiency and costs, simple planetary roller mechanisms must be designed as small as possible (including in transmission capability).
It seems these conflicting factors that have precluded brake mechanisms being easily added to frictional transmission type rotational driving apparatuses so that the adoption of brake mechanisms has finally been abandoned. This is ascribable to the general circumstances in this industry that using gear mechanisms, as mentioned before, consequently allows surer and less expensive fabrication rather than using roller mechanisms one or two ranks greater than required sizes.
The third aspect of the present invention has been achieved in view of the foregoing problems, and an object thereof is to combine a simple planetary roller mechanism, a motor, and a brake mechanism in optimum balance so as to provide a high cost-performance frictional transmission type rotational driving apparatus in which the braking capability is maintained while an increase in efficiency and a reduction on costs are achieved compatibly.
To describe the third aspect in more details, a typical brake mechanism, for example, sandwiches a brake ring with its brake shoes to exert braking power by means of frictional forces. For the brake mechanism to prevent the braking capacity from varying with revolution speeds, a relative difference between the static friction torque at rest (so-called retention torque) and the dynamic friction torque at revolutions is set to be as small as possible (the static friction torque being somewhat greater though). On the other hand, a simple planetary roller mechanism is not intended to produce a rotational load, but principally to make smooth rotations by applying a predetermined squeezing force to among the rollers. Accordingly, with respect to the transmission torque at rest (can also be called as retention torque), the transmission torque at revolutions may be xe2x80x9crelativelyxe2x80x9d low in considerable measure.
Then, the present inventors have focused attention on the fact that brake mechanisms and simple planetary roller mechanisms greatly differ from each other in the relative difference between the respective transmission (braking) capabilities at rest and at revolutions. That is, to well utilize this contrast, a new concept of xe2x80x9climit transmission torquexe2x80x9d which shows the transmission capability of a simple planetary roller mechanism at revolutions is introduced aside from the concept of xe2x80x9ccritical test torque Xdxe2x80x9d which represents the transmission capability at rest. This makes it possible to bring a simple planetary roller mechanism, a brake mechanism, and a motor into optimum balance based on those numeric values measured on an actual simple planetary roller mechanism.
More specifically, the above-described critical test torque Xd and limit transmission torque Xs, and the braking torques (static friction torque and dynamic friction torque) of a brake mechanism are set in the above-mentioned relations in magnitude. According to a frictional transmission type rotational driving apparatus set thus, the static friction torque obtainable from the brake mechanism can be surely transmitted via the simple planetary roller mechanism to a coupled mating machine at rest, so as to retain the mating machine stationary with reliability. Besides, at revolutions, the dynamic friction torque produced by the brake mechanism can be xe2x80x9csomewhatxe2x80x9d surely transmitted to the mating machine via the simple planetary roller mechanism, so as to reduce the revolution speed of the mating machine with reliability.
That is, in terms of its functions, the brake mechanism must stop the mating machine with reliability at the end, while the brake at revolutions originally involves the brake mechanism itself braking and sliding as well. Therefore, the simple planetary roller mechanism need not transmit the braking force to the mating machine xe2x80x9ccompletely,xe2x80x9d and may have its own potential for sliding. In other words, it is essential only that the rotational driving apparatus exerts predetermined braking power as far as the simple planetary roller mechanism can transmit, so as to lower the revolution speed of the mating machine with reliability. As the revolution speed on the mating-machine side gradually decreases, the limit transmission capability of the simple planetary roller mechanism increases gradually (has been verified by the inventors). Accordingly, it is possible to brake the mating machine without fail at the end, and the mating machine, once stopped, will not independently start to rotate.
Meanwhile, considering typical combinations of a motor and a brake mechanism, the constitution of the rotational driving apparatus according to the present invention has the optimum settings, for the transmission capability of the simple planetary roller mechanism, which is neither too great nor too small with respect to the driving capability of the motor at its rated revolution speed. That is, the sure braking functions as described above are exercised while sufficiently maintaining the properties of the frictional transmission type, namely, quiet and smooth transmission (high-efficient transmission) of rotational power.
As a consequence, according to the present invention, a high cost-performance rotational driving apparatus having three components, namely, a simple planetary roller mechanism, a brake mechanism, and a motor optimally set in capability balance is obtained.
In this rotational driving apparatus, the specific driving capability of the motor is preferably set as T less than Xs, where Xs is the limit transmission torque of the simple planetary roller mechanism and T is the rated torque of the motor. This setting makes it possible, in terms of driving capability as well as braking capability, to drive a mating machine coupled to this frictional transmission type rotational driving apparatus with yet higher reliability.
In the above-described invention, the limit transmission torque Xs of the simple planetary roller mechanism is set to fall within 0.65Ys less than Xs less than 3.4Ys, whereas it is preferably set as 0.8Ys less than Xs less than 1.7Ys, and best set so that Ys and Xs nearly equal to each other.
The nature, principle, and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.